The Longest Lunar Eclipse of the 21st Century

On July 27th, 2018 the world will see its first central lunar eclipse since June 15th, 2011. If you live in the Eastern Hemisphere, you’re in luck! Central lunar eclipses are much rarer than normal lunar eclipses, but what exactly is the difference?

A typical lunar eclipse occurs when the Moon passes through the Earth’s shadow, which comprises two components: the umbra and the penumbra. The umbra is the darkest part of the shadow, while the penumbra is the edge of the shadow. A lunar eclipse is different from a solar eclipse (like the one on August 21st, 2017), which occurs when the Earth passes through the tiny shadow created by the Moon. Since lunar eclipses happen in the Earth’s shadow, people in the hemisphere experiencing nighttime can observe lunar eclipses.

A map of where the central lunar eclipse will be visible. (Source: Sky and Telescope)

During a lunar eclipse, observers see a full Moon that begins to darken on one side as the Earth’s penumbra slowly covers its surface. The moving shadow makes it appear as if the Moon is rapidly going through its phases. Once the Moon enters the Earth’s umbra, the grayish-white surface of the Moon slowly transforms into a rusty reddish color due to a phenomenon known as Rayleigh Scattering.

Rayleigh Scattering has a daily effect on the Earth, which can be seen in the oranges, reds, and purples of sunsets and sunrises! With the Moon in the Earth’s shadow, any sunlight being reflected off the Moon had to first pass around the edge of the Earth and make it through our atmosphere. The visible light emitted by the Sun is white light, so when sunlight interacts with the Earth’s atmosphere, it is broken into different colors (wavelengths). Blue light is easily absorbed by our atmosphere (which is why the sky is blue), while red light passes through and travels to the Moon.

What makes this upcoming eclipse unique is the path the Moon will take through the Earth’s shadow. The umbra is quite large, and depending on the location of the Moon in its orbit, the Moon can take different paths through the shadow. A central eclipse occurs when a large portion of the Moon passes through the center of the umbra. During a central lunar eclipse, the apparent change of the Moon’s phases will be more noticeable, and the Moon will turn a darker shade of red.

A still image of the Moon as it passes through the shadow of the Earth (Source: Wikipedia)

A video of the Moon as it passes through the Earth's shadow. (Source: Wikipedia)

A path through the middle of the shadow also means that the Moon will take the longest route through the circular umbra of the Earth. A longer path means more time spent in the shadow, making central eclipses longer than other lunar eclipses. This year’s eclipse is especially unique, as the Moon will be at the point in its orbit farthest from the Earth (known as apogee). A Moon at apogee appears smaller in the sky, much like how mountains shrink when you get farther away. The combination of the Moon traveling through the middle of the Earth’s shadow and being at apogee, the Eastern Hemisphere will experience the longest central lunar eclipse in the 21st century: 103 minutes!